Ruby: allow Google::Protobuf::Map#to_h to accept a block

[bhang-stripe]

In Ruby, Google::Protobuf::Map is an Enumerable. From the Ruby documentation for Enumerable#to_h:

If a block is given, the results of the block on each element of the enum will be used as pairs.

This PR adds the ability to pass in a block and have the described behavior.

This is useful, otherwise we'd need to do something like call map and then to_h, which allocates intermediate results.

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[JasonLunn]

Before we can review this PR, please sign the CLA.

[bhang-stripe]

Before we can review this PR, please sign the CLA.

Done!